Getter-ion pump for producing and maintaining a high vacuum

ABSTRACT

A getter-ion pump for producing and sustaining a high vacuum in a vessel includes a pump housing in communication with the vessel. The pump housing includes spaced anode and cathode electrodes between which an arc is struck and maintained and the electrodes are in the form of coaxial cylinders one of which is constituted at least by two interconnected parts of different diameters such that along an axial extension which is at least greater than the spacing between the cylinders and at least 1.1fold increase in spacing between the cylinders is achieved.

United States Patent GETTER-ION PUMP FOR PRODUCING AND MAINTAINING AHIGH VACUUM 23 Claims, 3 Drawing Figs.

0.8. CI 417/49 Int. Cl. F04b 37/02 Field otSearch 4l7/48,49,

[56] References Cited UNITED STATES PATENTS 3,100,274 8/1963 Luftman etal.

3,394,874 7/l 968 Marshall Primary Examiner-Robert M. Walker Attorney-Pierce. Schefiler and Parker 31 7/49X 4 l 7/49X ABSTRACT: A getter-ionpump for producing and sustaining a high vacuum in a vessel includes apump housing in communication with the vessel. The pump housing includesspaced anode'and cathode electrodes between which an arc is struck andmaintained and the electrodes are in the form of coaxial cylinders oneof which is constituted at least by two interconnected parts ofdifferent diameters such that along an axial extension which is at leastgreater than the spacing between the cylinders and at least LI-foldincrease in spacing between the cylinders is achieved.

PATENTEDJUN BIB/l 35 3529 sum 2 or 3 Fig.2

INVENTQR MJ. SCHONHUBER ATENIED JUN 8 mm SHEET 3 BF 3 INVENTOI? M. J.SCHONHUBER GETTER-ION PUMP FOR PRODUCING AND MAINTAINING A HIGH VACUUMThis invention relates to a getter-ion pump for the production andmaintenance of a high vacuum by means of electrodes between which anelectric discharge occurs.

The invention is related from a standpoint of general subject matter tothe getter-ion pump structure disclosed in my copending application Ser.No. 750,442 filed Aug. 5, 1968, now Pat. No. 3,489,336, and in whichmeans are provided, which on switching on the pump, cause an arc to bestruck between a first electrode and a second electrode at a differentpotential for the purpose of forming at least one cathode spot,

in the region of which the surface of the cathodic getter material ismolten, said means including either an ignition pin and a startingdevice which momentarily short circuits the electrodes on switching on,or a switching device for producing a striking voltage between theelectrodes and in which further means are provided for maintenance ofthe are produced, so that the latter wanders about in the space formedbetween the electrodes and simultaneously the cathode spot on thesurface of the electrode moves about, said further means including aswitching device for the application of a low DC voltage to theelectrode, the space between the electrodes being connected with thereceiver via at least one opening allowing the passage of gas.

This getter-ion pump can be still further improved and then enables anactual pump housing to be dispensed with and enables wandering of thecathode spot to be kept within bounds in a reliable manner and alsoenables any atomization of the electrode insulating material to beinhibited in a reliable manner and at the same time inhibits to aconsiderable extent any atomization of the electrode itself into thereceiver, in addition to which the arrangement is made independent ofany coolant supply.

It is consequently proposed in accordance with the present inventionthat the electrodes shall consist mainly of at least approximatelycoaxial cylinders, the outer surface of the one and the inner surface ofthe other cylinder forming electrode surfaces, one cylinder at leastalso constituting the main portion of the pump housing, and that onecylinder at least shall consist of at least two interconnected portionsof different diameters, so that an at least l.lfold increase in spacingis achieved at least along an axial extension greater than the remainingspacing between the approximately coaxial cylinders.

In the accompanying drawings, FIGS. 1 to 3 show different embodiments ofthe invention:

FIG. 1 shows an embodiment in which the main electrode acting as cathodeand having two interconnected cathodic cylinder portions of differentdiameters is coaxially located within the main electrode or theappropriate cylinder acting as anode.

FIG. 2 shows an embodiment in which the main electrode connected as ananode and having two interconnected anodic cylinders of differentdiameters is located coaxially with the cathodic main electrode alsofunctioning as the pump housing and in which the gas passage or passagesleading to the cylinder is/are in the form of a pump connection orconnections in the cathode cylinder.

HO. 3 shows an appropriate electrical circuit for striking the arebetween the electrodes of the pump and subsequently maintaining it.

ln FIG. 1, 1 is the vacuum vessel-receiver for short. The getter-ionpump is mounted on this receiver. Said pump is built into the mainelectrode-the anode-consisting mainly of the cylinder 9 (or its outerwall 2 as the case may be) and of the disc 5 provided with gas passages,said electrode being sealed with a metal cover 3. lnto this vesselfunctioning as anode is built the cathodic main electrode having acathode cylinder 8 having two interconnecting portions 12, 13 ofdifferent diameters and the sealing disc 4 therefor. The are is intended to burn between these two main electrodes.

An auxiliary electrode 6 insulated electrically from the main electrodeby means of insulation 7 is also provided. Said auxiliary electrodeconsists partly at least of a very refractory metal, whereas, the mainelectrodes 4, 8 or 5, 9 respectively consist at least partly of gettermaterial or of a layer of getter material at least 0.5 mm. in thicknessapplied to a previously degasified base metal.

Part of the anode 5,9 is in a form of a screen, in order to facilitatepassage of gas residues from the receiver 1 into the pump. A part atleast of the cathode 4,8 is closely spaced to the anode 5, 9. Theauxiliary electrode 6 at this place is located close to the mainelectrode 4, 8 functioning as the cathode. A high voltage pulse is firstimpressed on the auxiliary electrode so that an arc is struck betweensaid auxiliary electrode and the cathode 4, 8 even when a high vacuumobtains in the pump, with the result that at least one cathode spot isformed on the main cathode 4,8. The rectifier circuit 31 (FIG. 3) isclosed via the cathode spot already existing on the cathode 4, 8 owingto the auxiliary are through the vapor thus evolved from the electrodeand its consequent ionization and the existing ionization of the spacebetween the main electrodes 4, 8 and 5, 9 and a heavy current of severalhundred amperes can flow from the low voltage source, but which must besuitably energized, in order to sustain a powerfullow voltage arc. Thisprocedure increases the number of cathode spots at least in the highandultrahigh vacuum range, since if the current strength exceeds the valueset by the electrode material, a cathode spot will divide up into two ormore spots. At the same time, migration of the cathode spot into part ofthe electrode with increased electrode spacing will increase the lengthof the individual arc plasma columns. Consequently, the volume of theare which can be sustained and the strength and the current strength ofwhich becomes increased exceeds the value of the are originally struckand consequently also the volume of the gas flowing through the are, sothat an adequately large pumping speed can be achieved even in a highvacuum.

Since the getter-ion arc pump can also continue to act even in theintervals when switched off as a sorption pump and can effectively pumpnonrare gases by means of the active surface films deposited from thevaporization of the electrode, the surfaces facing the cathode shouldfor this purpose be as large as possible, by for example designing theanode 5, 9 which is provided partly at least with gas passages and ispartly at least in the form of a screen 5 with oblique slots and havingstays 10 with optional liquid cooling, the gas passages being fittedwith a system of cooled baffles 11 on the side facing the receiver.

As a condition determining adequacy of pumping capacity, it is of quitedecisive importance that on the one hand as large as possible anelectrode surface should be available over which the cathode spot canwander, which is achieved according to my aforesaid copendingapplication by combining the disc-shaped electrode4, 5 with the coaxialcylinders 8, 9 and that on the other hand means are available forreliably limiting the wandering of the cathode spot, in order withoutfail to keep vapors arising from the electrode away from the electrodeinsulation 17.

Consequently, in accordance with the invention, the electrode surfaceformed by the coaxial cylinders 8, 9 is to be limited by the fact thatat least one of the two cylinders, for example, cylinder 8 in FIG. 1,consists of at least two interconnected parts of different diameter 12,13, so that along an axial extension at least, which is greater than theremaining spacing between the coaxial cylinders, at least a l.lfoldincrease in spacing 14 between the cylinders is achieved. More preciselythis increase in spacing is achieved in such a manner that the totalvoltage consisting of the voltage drop at the cathode plus the productof the arc voltage gradient and electrode spacing is greater than therated voltage of the low voltage source less the DC voltage drop whenthe pump is operating at its rated current discharge in the feedcircuit. The effect thereof is the automatic extinguishing of anycathode spot which might wander over the cylindrical portion 13 of.reduced diameter for the purpose of such increased spacing or within thecorresponding zone 14 respectively of the pump, and the prevention ofany wandering of the cathode spot into the gap 15 and labyrinth 17protecting the electrode insulation, which would destroy the functioningof the getter-ion pump. Other cathode spots still remaining within thepermitted electrode area 4, 12 subdivide automatically at the same rateas cathode spots wander off said permitted area, so that by this means,given constant current strength, the total number of cathode spotsremains constant and continuous pump operation is assured.

A flanged projection 20 is provided on the inner electrode cylinder 8 or13 as the case may be facing the active electrode area for the purposeof producing the narrow gap 15 or the labyrinth l6 and thus reliablyscreening off the electrode insulation 17.

If it is desired to intensify the pumping action of a getter-ion arcpump, the cylindrical electrode 8, 9 can be cooled, for example, bycirculating a liquid coolant in the interior of the electrode. In manycases, however, such circulation of liquid coolant is either undesirableor complicated. Consequently, a mode of cooling is proposed in FIGS. 1and 2, according to which a portion at least of the interior ofcylindrical electrodes 8, 9 forms a sealedoff cavity 18, 19 containing acoolant in both liquid and vapor phase, a portion at least of saidcavity having a surface structure producing a capillary action.

Known examples of surface structures having a capillary action for theinternal walls of such an electrode cavity are fine grooves, a porousvlayer or a fine mesh screen. This surface structure is not depicted inFIGS. 1 or 2. If now a quantity ofa liquid coolant which is sufficientto fill the capillaries is charged into the cavity and the boiling pointthereof lies between 50 C. and 350 C, the electrode temperature can inthe main be maintained below the boiling point of the liquid coolanteven when large quantities of heat are evolved from the cathode spots,and once gases are occluded, absorbed or absorbed on the surfaces of theelectrodes said gases remain trapped on the electrode surfaces 4 and 8or in the interior thereof. Examples of suitable liquid coolants arewater or organic liquids, for example, the suitability of which can beenhanced by adding a wetting agent to the liquid coolant. A lowering ofthe boiling point of the coolant may be achieved by partly evacuation ofthe cavities 18, 19 in the electrode. The coolant evaporating in theactive surface regions of the electrodes recondense in the zones of thecavity remote from these regions and the effect of the surface structurewith capillary action is to cause the condensate returned to theportions of the electrode to be cooled independent of the position andmanner in which the getter-ion pump has been fitted up.

The portions indicated as 21, 22 constitute cooling ribs, whichdissipate the heat evolved during the condensation of the liquid coolantto the ambient atmosphere.

In the embodiment according to FIG. 1, a low voltage electricalinsulator 24, is provided between the receiver 1 and the connectingflange 23 of the getter-ion pump similar to that between the flanges 3and 25. The connections between the flanges 3 and 25 and between flange23 and the receiver 1 is made in a vacuum-tight manner. Moreover, theinsulator 7 for the lead-in 26 of the auxiliary electrode, which may,for example, consist of an alumina tube metallized on both sides, isbonded in a vacuum-tight manner-on the one side to the connecting flange26 for the auxiliary electrode and on the other side to the cathodecylinder 8. In order to screen the auxiliary electrode insulator fromatomized metal deposits, provision is made for a gap between theinsulator surface and the auxiliary electrode, but this is not shown inFIGS. 1 and 2.

FIG. 2 shows an embodiment of pump, in which the anodically connectedmain electrode 9 consisting mainly of two part cylinders 12, 13 ofdifferent diameters, is located coaxially within the cathodicallyconnected substantially cylindrical main electrode 8, which forms alarge part of the pump housing. The circular plates 4, 5 closing thefront ends of cylinders 8, 9 form the remaining portion of the mainelectrodes. The gas passage or passages leading to the receiver 1 is/orare mounted in the cathode cylinder electrode 8, at a place to whichwandering of a cathode spot is precluded by the increase in spacingbetween cylinders 8, 9 according to the invention. The flangedprojection 20, on the anodic cylinder electrode 9 forms an arrangementwhich does not impede the passage of gas from the receiver for achievinga narrow gap 15 or labyrinth l6 and consequently reliable screening ofthe electrode insulation 17 from any atomized metallic deposits. Anauxiliary electrode 6 electrically insulated by insulators 7 is mountedwith close spacing in the wall of the cathodic cylinder electrode 8, theshort are first struck immediately on ignition of the pilot arc beingdrawn out as it finally passes over and burns between the mainelectrodes.

This embodiment according to FIG. 2 can also be provided with a movableignition pin instead of the auxiliary electrode. This means for strokingthe pilot arc can, with advantage, also be mounted on the disc-shapedpart electrode 4.

In order to facilitate the striking of the arc, the point of theauxiliary electrode or the opposite main electrode may consist at leastpartly of a radioactive isotope, which emits aor B rays.

However, the arc may be struck in simple manner in the case of thisembodiment without the aid of the auxiliary electrode 6 shown in FIG. 2(or an ignition pin as the case may be) for example, by mounting thedisc-shaped cathodic electrode 4 so as to be movable within the cylinder8 so that at least when switching on the pump the electrodes are atleast partly brought in contact with one another, and a starting deviceenables the electrodes to be brought into contact and then separated,and allows the spacing between them to be increased.

FIG. 3 shows the electrical circuit. A switching device for striking thearc includes an impulse transformer 34, a condenser 36 and a blockingrectifier 39. A further switching device as a means for sustaining thearc comprises a rectifier transformer 30, and a rectifier 31. Anauxiliary electrode 6-, serves as a means for decoupling between thehigh tension source serving to strike the arc and the low voltage sourceserving to sustain the are, said auxiliary electrode being mounted inclosely spaced relationship to the cathodic main electrode 4, 8 of thegetter-ion pump. i

The mode of operation is as follows: Switches 29 and 38 are firstclosed, whereupon condenser 36 is charged from the mains via therectifier 39. On depressing the switch button 35, condenser 36discharges through the primary winding of the ignition coil 34, whichfurnishes a high tension pulse at the auxiliary electrode 6 and thecathodic main electrode 4, 8 of the getter-ion pump for the striking ofan are or the production of a cathode spot at least. By this means, thespace between the main electrodes 4, 8 and 5, 9 is preionized, so thatthe circuit of the rectifier 31 is closed via the cathode spot alreadyexisting at the cathode due to the pilot arc and a heavier current for apowerful arc discharge including a plurality of cathode spots can beproduced.

At the end of the pumping operation, switch 29 is opened again. Openingand closing of switch 29 and the operation of the push button 35 canalso be carried out automatically by using a pressure monitoring devicewhich comes into operation when the vacuum in the apparatus deterioratesand switches off when the desired vacuum is reestablished. Thisautomatic device is not shown in FIG. 3.

Iclaim:

I. In a getter-ion pump for the production and maintaining of ahigh-vacuum in a receiver and having main electrodes between which anelectric discharge occurs and in which means are provided which onswitching on the pump strike an are between two electrodes at adifferent potential for the purpose of the production of at least onecathode spot in the region of which the surface of the cathodeconsisting of getter material is molten, wherein further means includinga switching device for applying a low DC voltage to the main electrodesare provided for sustaining the are so produced so that the arc wandersin the space formed between the main electrodes and simultaneously thecathode spot wanders over the surface of the cathode, and wherein thespace between the main electrodes is connected by way of at least onegas passage with the receiver, the improvement wherein said mainelectrodes are comprised substantially of at least approximately coaxialcylinders, the outer surface of one and the inner surface of the othercylinder constituting electrode surfaces, wherein one cylinder at leastalso forms a major part of the pump housing, and wherein at least one ofsaid cylinders is constituted at least by two interconnected parts ofdifferent diameters such that along an axial extension at least which isgreater than the residual spacing of said coaxial cylinders an atleastl.l--fold increase in spacing between said cylinders is achieved.

2. A getter-ion pump as defined in claim 1 wherein said gas passage isestablished by a passage through at least one of said electrodes.

3. A getter-ion pump as defined in claim 1 wherein at least one of saidelectrodes is constituted at least partly in the form of a screen.

4. A getter-ion pump as defined in claim 3 wherein the screen portion ofsaid electrode is formed with oblique slots.

5. A getter-ion pump as defined in claim 1 and which further includes abaffle system located in front of at least one gas passage at the sideof the receiver.

6. A getter-ion pump as defined in claim 5 and which further includesmeans for liquid-cooling of said baffie system.

7. A getter-ion pump as defined in claim 1 wherein said means forstriking the arc comprises a starter having an optionally used coilwhich moves at least one of said electrodes at least on switching ontowards said second electrode having a counter potential, during which aconnection and separation and an increase in electrode spacing isachieved.

8. A getter-ion pump as defined in claim 1 and which further includes atleast one auxiliary electrode closely spaced to the cathodic mainelectrode as a means for decoupling between the high voltage whichserves to strike the arc and the low DC voltage which serves to maintainthe are.

9. A getter-ion pump as defined in claim 8 wherein said auxiliaryelectrode or the main electrode facing said auxiliary electrode isconstituted at least in part by a radioactive isotope.

10. A getter-ion pump as defined in claim 1 and which further includes aswitching device for striking an are between one of said main electrodesand at least one auxiliary electrode, said device serving to strike apilot arc with at least one cathode spot on the main electrode by way ofan impulse transformer between the electrode connected as a cathode bothin the main and also in the pilot discharge paths, said auxiliaryelectrode being located in the immediate vicinity of the cathode, saidcathode being molten within the range of said cathode spot and saidpilot are having a current strength such that the arc is caused by wayof a further switching device to burn between said main electrodes andthere be maintained.

11. A getter-ion pump as defined in claim 1 and which further includesmeans for cooling at least one of said main electrodes by a liquidcoolant from a cooling device connected to said main electrode by way ofsupply and discharge pipes for the coolant.

12. A getter-ion pump as defined in claim 1 wherein a surface of atleast one of said main electrodes forms a wall portion of a sealed offcavity which contains both a cooling fluid and also the vapor phasethereof, at least one portion of said surface being structured toestablish a capillary action.

13. A getter-ion pump as defined in claim l2 wherein the surface of saidmain electrode defining said sealed-off cavity is provided with finegrooves to establish the capillary action.

14. A getter-ion pump as defined in claim 12 wherein the surface of saidmain electrode defining said sealed off cavity is constituted by aporous layer to establish the capillary action.

15. A getter-ion pump as defined in claim 12 wherein the surface of saidmain electrode defining said sealed-off cavity is constituted by a finemesh screen to establish the capillary action.

16. A getter-ion pump as defined in claim 12 wherein said cooling fluidis a liquid having a boiling point between 50 C. and 300 C. atatmosphere pressure.

17. A getter-ion pump as defined in claim 16 wherein said cooling liquidis constituted by water.

18. A getter-ion pump as defined in claim 16 and-wherein said coolingliquid includes a wetting agent added thereto.

19. A getter-ion pump as defined in claim 16 wherein said sealed-offcavity defined by said main electrode also contains an extraneous gaswhich is noncondensable at ambient temperature and the partial pressureof which is less than atmospheric at ambient temperature.

20. A getter-ion pump as defined in claim 1 wherein a surface of atleast one of said main electrodes forms a wall portion of a sealed-offcavity which contains both a cooling fluid and also the vapor phasethereof, at least one portion of said surface being structured toestablish a capillary action, and wherein said main electrode whichforms the wall portion of said sealed-off cavity is provided withcooling ribs.

21. A getter-ion pump as defined in claim 1 wherein at least one of saidmain electrodes is constituted at least partly by getter material.

22. A getter-ion pump as defined in claim 1 wherein at least one of saidmain electrodes is constituted by a thermally conductive base metal towhich is applied a layer of getter material.

23. A getter-ion pump as defined in claim 22 wherein said gettermaterial has a thickness of at least 0.5 mm.

1. In a getter-ion pump for the production and maintaining of ahigh-vacuum in a receiver and having main electrodes between which anelectric discharge occurs and in which means are provided which onswitching on the pump strike an arc between two electrodes at adifferent potential for the purpose of the production of at least onecathode spot in the region of which the surface of the cathodeconsisting of getter material is molten, wherein further means includinga switching device for applying a low DC voltage to the main electrodesare provided for sustaining the arc so produced so that the arc wandersin the space formed between the main electrodes and simultaneously thecathode spot wanders over the surface of the cathode, and wherein thespace between the main electrodes is connected by way of at least onegas passage with the receiver, the improvement wherein said mainelectrodes are comprised substantially of at least approximately coaxialcylinders, the outer surface of one and the inner surface of the othercylinder constituting electrode surfaces, wherein one cylinder at leastalso forms a major part of the pump housing, and wherein at least one ofsaid cylinders is constituted at least by two interconnected parts ofdifferent diameters such that along an axial extension at least which isgreater than the residual spacing of said coaxial cylinders an atleast1.1-fold increase in spacing between said cylinders is achieved. 2.A getter-ion pump as defined in claim 1 wherein said gas passage isestablished by a passage through at least one of said electrodes.
 3. Agetter-ion pump as defined in claim 1 wherein at least one of saidelectrodes is constituted at least partly in the form of a screen.
 4. Agetter-ion pump as defined in claim 3 wherein the screen portion of saidelectrode is formed with oblique slots.
 5. A getter-ion pump as definedin claim 1 and which further includes a baffle system located in frontof at least one gas passage at the side of the receiver.
 6. A getter-ionpump as defined in claim 5 and which further includes means forliquid-cooling of said baffle system.
 7. A getter-ion pump as defined inclaim 1 wherein said means for striking the arc comprises a starterhaving an optionally used coil which moves at least one of saidelectrodes at least on switching on towards said second electrode havinga counter potential, during which a connection and separation and anincrease in electrode spacing is achieved.
 8. A getter-ion pump asdefined in claim 1 and which further includes at least one auxiliaryelectrode closely spaced to the cathodic main electrode as a means fordecoUpling between the high voltage which serves to strike the arc andthe low DC voltage which serves to maintain the arc.
 9. A getter-ionpump as defined in claim 8 wherein said auxiliary electrode or the mainelectrode facing said auxiliary electrode is constituted at least inpart by a radioactive isotope.
 10. A getter-ion pump as defined in claim1 and which further includes a switching device for striking an arcbetween one of said main electrodes and at least one auxiliaryelectrode, said device serving to strike a pilot arc with at least onecathode spot on the main electrode by way of an impulse transformerbetween the electrode connected as a cathode both in the main and alsoin the pilot discharge paths, said auxiliary electrode being located inthe immediate vicinity of the cathode, said cathode being molten withinthe range of said cathode spot and said pilot arc having a currentstrength such that the arc is caused by way of a further switchingdevice to burn between said main electrodes and there be maintained. 11.A getter-ion pump as defined in claim 1 and which further includes meansfor cooling at least one of said main electrodes by a liquid coolantfrom a cooling device connected to said main electrode by way of supplyand discharge pipes for the coolant.
 12. A getter-ion pump as defined inclaim 1 wherein a surface of at least one of said main electrodes formsa wall portion of a sealed off cavity which contains both a coolingfluid and also the vapor phase thereof, at least one portion of saidsurface being structured to establish a capillary action.
 13. Agetter-ion pump as defined in claim 12 wherein the surface of said mainelectrode defining said sealed-off cavity is provided with fine groovesto establish the capillary action.
 14. A getter-ion pump as defined inclaim 12 wherein the surface of said main electrode defining said sealedoff cavity is constituted by a porous layer to establish the capillaryaction.
 15. A getter-ion pump as defined in claim 12 wherein the surfaceof said main electrode defining said sealed-off cavity is constituted bya fine mesh screen to establish the capillary action.
 16. A getter-ionpump as defined in claim 12 wherein said cooling fluid is a liquidhaving a boiling point between 50* C. and 300* C. at atmospherepressure.
 17. A getter-ion pump as defined in claim 16 wherein saidcooling liquid is constituted by water.
 18. A getter-ion pump as definedin claim 16 and wherein said cooling liquid includes a wetting agentadded thereto.
 19. A getter-ion pump as defined in claim 16 wherein saidsealed-off cavity defined by said main electrode also contains anextraneous gas which is noncondensable at ambient temperature and thepartial pressure of which is less than atmospheric at ambienttemperature.
 20. A getter-ion pump as defined in claim 1 wherein asurface of at least one of said main electrodes forms a wall portion ofa sealed-off cavity which contains both a cooling fluid and also thevapor phase thereof, at least one portion of said surface beingstructured to establish a capillary action, and wherein said mainelectrode which forms the wall portion of said sealed-off cavity isprovided with cooling ribs.
 21. A getter-ion pump as defined in claim 1wherein at least one of said main electrodes is constituted at leastpartly by getter material.
 22. A getter-ion pump as defined in claim 1wherein at least one of said main electrodes is constituted by athermally conductive base metal to which is applied a layer of gettermaterial.
 23. A getter-ion pump as defined in claim 22 wherein saidgetter material has a thickness of at least 0.5 mm.